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The present invention relates generally to accelerometers, and more particularly to high precision accelerometers constructed with microwave resonant cavities.
High precision accelerometers are used for various commercial and military purposes, including aircraft navigation and missile guidance. One type of high precision accelerometer known in the art is the pendulous gyroscopic accelerometer, which is based on a rebalance mechanism. An unbalance is created, for example by adding a pendulous mass along a spin axis. An input acceleration creates a torque, which is counterbalanced by a torque in the opposite direction resulting from the rotation of the gyroscope about its input axis. The velocity of rotation of the gyroscope is used to determine the acceleration being sensed by the accelerometer. While the pendulous gyroscopic accelerometer can meet the performance standards required in strategic missile applications, it is an expensive and complex device, and has a large number of parts that require extensive precision machining and assembly time. The pendulous gyroscopic accelerometer is therefore not mass-producible as a small, inexpensive device adapted for commercial applications, and is incompatible with micromachining technologies.
U.S. Pat. No. 5,351,541 discloses a microwave resonator accelerometer which includes a structure defining a cylindrical microwave resonant cavity. The-microwave resonant cavity is bounded at one end by a flexible member. The flexible member supports a proof mass so that the proof mass is moveable along a sensing axis. U.S. Pat. No. 5,292,569 discloses a flexible member for use in an accelerometer. An input acceleration force applied to the proof mass causes a displacement of the proof mass and the supporting flexible member along the sensing axis. The displacement of the flexible member changes the dimensions of the cavity, resulting in a change in the resonant frequency of the cavity. The shift in resonant frequency is used to determine the acceleration force applied to the proof mass. The microwave resonator accelerometer disclosed in U.S. Pat. No. 5,351,541 is simpler, more rugged and much less expensive than prior art pendulous gyroscopic accelerometers, while meeting a comparable standard of performance.
The acceleration-based variance in the resonant frequencies of the resonant cavities in U.S. Pat. No. 5,351,541 has, however, a low sensitivity to the displacement of the proof mass when applied to miniaturized proof masses, i.e. a frequency shift is not readily detectable for small displacements of miniaturized proof masses. Typically, miniaturized proof masses are less than 1 cm2 in area.
Further, the microwave resonator accelerometer disclosed in U.S. Pat. No. 5,351,541 does not lend itself to miniaturization or micromachining in general, because of its cylindrical shape. Cylindrically shaped cavities have fairly large sizes at a given frequency of operation, typically about one half the wavelength of the microwave signal at the frequency of operation. If miniaturization of the cylindrical resonant cavity accelerometer is attempted, the resonant frequencies of the cavity become too high for reliable detection and processing, rendering such miniaturization impracticable with currently available electronic instrumentation.
It is therefore an object of the present invention to provide a microwave resonant cavity accelerometer that has a significantly improved sensitivity to an acceleration, as compared to prior art microwave resonant cavity accelerometers. It is another object of the present invention to provide a microwave resonant cavity accelerometer that can be fabricated using micromachining processes.
The present invention features an improved microwave resonant cavity accelerometer that achieves an improvement in sensitivity by a factor of about 100 over the prior art when applied to miniaturized proof masses that typically are less than 1 cm2 in area. In overview, the accelerometer includes (i) a reentrant microwave resonant cavity characterized by a nominal resonant frequency and bounded in part by a proof mass positioned along a sensing axis, (ii) a coupler for coupling an electromagnetic signal into the cavity substantially at the nominal resonant frequency of the cavity, and (iii) means for detecting electromagnetic energy in the cavity and determining a frequency of the detected energy. A displacement of the proof mass along the sensing axis in response to an acceleration force changes the dimensions of the cavity so as to establish a resonant frequency for the cavity that varies as a function of the acceleration force. The shift in resonant frequency provides a measure of the acceleration.
A microwave resonant cavity constructed according to the present invention includes a rigid body member, a rigid end member, and a flexible end member. The rigid body member is open-ended and hollow, and is disposed about a central void region. The body member extends along a sensing axis from a first end to a second end, and includes an electrically conductive inner wall that bounds in part the void region. The rigid end member extends transverse to the sensing axis, and extends from the body member across the void region at the first end. The rigid end member includes an electrically conductive rigid wall that bounds in part the void region. The flexible end member extends transverse to the sensing axis, and extends from the body member across the void region at the second end. The flexible member includes an electrically conductive flexible wall that bounds in part the void region.
A proof mass is positioned along the sensing axis, and is supported by the flexible wall. The proof mass has an electrically conductive outer surface that bounds in part the void region. The outer surface of the proof mass extends from the flexible wall toward the rigid wall. A distal portion of the outer surface of the proof mass establishes a capacitive gap between the outer surface and the rigid wall. The capacitive gap is relatively narrow in the direction of the sensing axis. An annular region of the flexible wall between the proof mass and the body member establishes an inductive gap between the annular region and the rigid wall. The inductive gap is relatively large in the direction of the sensing axis.
The central void region bounded by the outer surface of the proof mass, the annular region of the flexible wall, and by the walls of the body member and the rigid end member forms a reentrant type resonant cavity that is characterized by a resonant microwave frequency.
An accelerometer constructed according to the present invention includes a reentrant type resonant cavity as described above. In a preferred embodiment, the accelerometer is a dual-cavity accelerometer that includes first and second complementary reentrant microwave cavities, each characterized by a nominal resonant frequency. A hollow, rigid cavity housing encloses a central void region. The cavity housing includes a plurality of electrically conductive inner walls bounding the central void region. A flexible, nominally planar member extends from an inner wall of the housing across the void region to an opposite inner wall. A proof mass is positioned along a sensing axis, and is disposed on and supported by the flexible member, so that the proof mass and the flexible member divide the central void region into a first void region extending from a first side of the flexible member and a second void region extending from a second side of the flexible member. An outer surface of the proof mass bounds in part the first and second void regions.
First and second portions of the outer surface of the proof mass establish first and second capacitive gaps between each portion and an electrically conductive inner wall that is disposed adjacent to and across from the portion. The capacitive gaps are relatively narrow in a direction of the sensing axis. An annular inductive gap surrounds each capacitive gap. The inductive gaps are relatively wide in the direction of the sensing axis.
The first and second void regions form first and second complementary reentrant microwave resonant cavities, each cavity being characterized by a nominal resonant microwave frequency. Preferably, the first and second reentrant microwave resonant cavities are tuned to the same nominal resonant frequency. In one embodiment, the nominal resonant frequencies of the first and second reentrant cavities are less than about 3 GHz.
One or more couplers couples an electromagnetic signal into each cavity, substantially at the nominal resonant frequency of the cavity. In one embodiment, the couplers include coaxial leads. Upon coupling of the electromagnetic signal into the cavity, the electric field within the cavity is substantially concentrated within the narrow capacitive gap, whereas the magnetic field within the cavity is substantially concentrated within the relatively wide inductive gap.
In response to an acceleration force along the sensing axis, the proof mass differentially changes the dimensions of each cavity, and establishes a resonant frequency for each resonant cavity which varies as a function of the acceleration force. The shift in resonant frequency is measured to determine the acceleration of the proof mass.
The accelerometer includes means for detecting electromagnetic energy in each cavity, and for determining the frequency of the detected energy. The means for determining the frequency of the detected energy includes means for discriminating, in response to a reflected microwave signal from the cavity, a frequency shift in the resonant frequency of each cavity.
In one embodiment, the means for determining the frequency of the detected energy includes a phase locked loop circuit. The phase locked loop circuit may include a signal source that generates an input microwave signal substantially at the nominal resonant frequency of the cavity. Preferably, the signal source is a voltage-controlled signal source whose frequency may be adjusted. The phase locked loop circuit may also include a phase discriminant circuit that discriminates a phase shift between the input microwave signal and a microwave signal reflected from the resonant cavity, and generates an indicator signal representative of the phase shift. The phase locked loop circuit may also include a feedback circuit that feeds the indicator signal back to the signal source. Upon feedback of the indicator signal, the signal source generates an output signal that varies in frequency according to the displacement of the proof mass induced by the acceleration force. The means for determining the frequency of the detected energy may also include a frequency counter that counts a frequency of the frequency varying output signal, and generates digital signals representative of the acceleration of the proof mass.
The accelerometer of the present invention may be fabricated using micromachining techniques. In one embodiment of the present invention, a micromachined accelerometer includes a miniaturized proof mass having a surface area less than about 1 cm2. In one embodiment, an inner radius of each cavity is less than about 0.50 cm, and an outer radius of each cavity is less than about 1.0 cm. In one embodiment, a micromachined accelerometer constructed according to the present invention is monolithic, namely the proof mass, the rebalance electrodes, and springs are etched upon a single substrate. In one embodiment, the monolithic accelerometer has an in-plane structure, namely the motion of the proof mass has a displacement vector parallel to a planar surface of the substrate.
In one embodiment, the resolution of the accelerometer of the present invention is about 1.1xc3x9710xe2x88x927 g, in 1 Hz bandwidth. The sensitivity of the accelerometer is about 1.3xc3x9710xe2x88x927 g/Hz. An improvement in sensitivity by a factor of about 100 is thereby achieved over the prior art, for miniaturized proof masses.
The present invention also features a method for measuring an acceleration induced by an acceleration force acting on a proof mass. The method includes the step of providing a reentrant microwave resonant cavity having a capacitive narrow gap and an inductive wide gap surrounding the capacitive gap, and having a nominal resonant frequency. The method further includes the step of coupling a microwave signal into the reentrant resonant cavity substantially at the nominal resonant frequency. The method further includes the step of inducing, in response to the acceleration force, a displacement of a proof mass that changes the dimensions of the reentrant resonant cavity. The method further includes establishing a resonant frequency of the reentrant resonant cavity which varies as a function of the acceleration force. The method further includes measuring a frequency shift in the resonant frequency to determine the acceleration of the proof mass.